Bilateral permanent congenital hearing loss is estimated to affect 1-3 neonates per 1,000 live births worldwide. A genetic basis is causal in approximately 50% of the individuals. Treatment options for congenitally deaf or hearing impaired children are limited in scope and efficacy and typically involve hearing aids or cochlear implants. Neither intervention fully restores the richness of native hearing. The conceptual basis of this proposal is to define fetal therapies that correct defects in gene expression or function prior to the onset of overt pathogenic changes in the sensory epithelia of the developing inner ear. We use the mouse as a model system because an ever-expanding array of natural and induced genetic mutations exists that serve as accurate paradigms for human inner ear dysfunction. Our technical approach relies on experimental embryology, a palette of surgical, imaging, microinjection and molecular techniques that permit access to the developing mouse inner ear in utero and enable genetic manipulation of precursor cells that give rise to the auditory and vestibular sensory structures. In this proposal, we aim: 1) to restore auditory function in a mouse model defective in synaptic transmission~ and 2) to restore auditory and vestibular function in a mouse model of Usher syndrome. A missense mutation in the human SLC17A8 gene that encodes vesicular glutamate transporter-3 (VGLUT3) has been associated with an autosomal-dominant form of progressive, high-frequency nonsyndromic deafness. VGLUT3 is selectively localized to inner hair cells in the cochlea and loads the excitatory amino acid neurotransmitter glutamate into synaptic vesicles. The VGLUT3 knockout mouse is born deaf due to the inability of inner hair cells to release glutamate at the afferent nerve terminals. We hypothesize that virus-mediated gene transfer of VGLUT3 to otic epithelial precursors will force VGLUT3 expression in inner hair cells and restore synaptic transmission and hearing in the VGLUT3 knockout mouse. Usher syndrome is the leading genetic cause of combined deafness and blindness and is associated with 13 loci and 10 genes in humans. Three distinct clinical forms are differentiated by the time of onset and severity of auditory, vestibular, and visual dysfunction. Usher syndrome type 1 is the most severe with bilateral profound hearing loss and balance difficulty at birth and retinitis pigmentosa present by early adolescence. The USH1 gene encodes the protein harmonin that is expressed in the sensory hair cell bundle and the ribbon synapse. A single base pair mutation in the harmonin gene creates a cryptic splice site resulting in a frameshift and truncated harmonin protein. The USH1C mouse mutant is born deaf, uncoordinated, and develops vision impairment. We hypothesize that fetal administration of an antisense oligonucleotide will correct harmonin messenger RNA splicing and restore auditory and vestibular function in the USH1C mouse mutant. We are optimistic that the proposed studies will establish the first fetal therapies for congenital deafness and vestibular dysfunction and will help inform translation of these approaches to the clinic.